1. Field of the Invention
This invention relates to a screw driving machine which hammers a screw and then screws it. More particularly, the invention relates to a contact arm locking mechanism in a screw driving machine in which a contact arm is locked by the locking mechanism on its half way.
2. Related Art
A screw driving machine, as disclosed by Japanese Utility Model Application Laid-Open No. Hei. 6-36774, has a driver which performs a screw hammering operation in a direction of axis, and a screwing operation around axis. The driver is driven by compressed air to hammer a screw into a material into which a screw is to be screwed (hereinafter referred to merely as "a work", when applicable), and screw the screw into the work. If, in the screw hammering operation, the stem of the screw penetrates the work, then the following screw screwing operation means nothing, and the screw has little extracting resistance from the work. Thus, in order to sufficiently hold the screw screwed in the work, it is essential to hammer the screw into the work to a predetermined depth according to the thickness of the work.
Hence, heretofore, as shown in FIGS. 6 through 8 of the Specification of the aforementioned Japanese Utility Model Application, in order to make the screw hammering stroke of the driver to end at a predetermined position, the following hammering depth control mechanism is employed: A contact member which abuts against the work is arranged slidably along a nose section from which screws are ejected. In hammering a screw into the work, the contact member is held at a predetermined stop position to hold the nose section at a predetermined distance (or at a predetermined height) from the surface of the work, thereby to adjust the screw hammering depth.
In the above-described hammering depth control mechanism, a rotary arm integral with the rotary shaft of a pinion-rack mechanism is engaged with the upper end of the contact member. When the trigger lever is operated, the screw hammering operation is started. Thereafter, the pinion-rack mechanism is operated. In association of the operation of the rack, the rotary arm is rotated to release the contact member, so that the screw screwing operation is carried out. The upper end of the contact member is moved to a retracting position located above beyond the position of the rotary arm. When the trigger lever is released after the screw screwing operation, the rack is reradiated, and the rotary arm is rotated to the initial position.
However, the rotary arm, while rotating to the initial position, may strike against the side surface of the contact member, thus obstructing the returning of the contact member. If the contact member is not returned to the initial position, in the next screw hammering operation, it is impossible to hammer the screw to the predetermined depth, and the pinion gear coaxial with the rotary arm cannot be returned to the initial position. Hence, in the next screw screwing operation, the screw is not sufficiently rotated, so that the screw is not sufficiently held in the work.